As a method for applying print to the surface of a non-flat article, a method is generally adopted which includes floating, on the surface of water, a transfer sheet having a print layer with the printed surface facing up to fully swell the sheet and then pressing an article downwardly towards the inside of water to transfer the print layer to the surface of the article (see, for example, JP 51-21914 A and JP 54-33115 A). For such transfer sheets, usually, films prepared from water-soluble or water-swellable resin such as a polyvinyl alcohol polymer (henceforth, a polyvinyl alcohol polymer is also abbreviated to as “PVA” and a polyvinyl alcohol polymer film is also abbreviated as “PVA film”) have been used as base films. A base film for use in this application is required to be superior in printability, to swell when being floated on the surface of water, and to have a property of clinging to an article (that is, clinginess). Hydraulic transfer printing base films which meet such requirements were proposed in the past (see, for example, JP 54-92406 A and JP 54-150208 A). However, PVA films have a property to swell and extend gradually when being floated on water. Therefore, when a print layer is formed on a base film formed of PVA and transfer printing is carried out therewith, the print layer extends together with the base film swollen on the surface of water and a print pattern transferred to an article may differ from the original print pattern formed on the base film and, in particular, the print pattern may extend to blur. Thus, usual PVA films have a problem that it is impossible to transfer a clear and high-definition print pattern.
Although printing plates have been made with patterns scaled down, it has not been possible, even in such cases, to fully suppress blur of print patterns caused by extension of a transfer sheet. In order to solve this problem, proposed is a method comprising floating a transfer sheet on the surface of water to swell the sheet until the extension stress disappears, then scaling the sheet down gradually in its transverse direction, and transferring the patterns to an object while maintaining the sheet at a prescribed width (JP 4-308798 A). In addition, as an attempt to improve a base film itself, a thin film for transfer printing having a thickness of from 10 to 50 μm and made of PVA and a specific natural gum-based mucilage, wherein a swelling extensibility is 1.35 or less, which is an area magnification after a lapse of a time three times as long as a swelling time (that is, a time needed, when a thin film is floated on the surface of water at 25° C., until wavy wrinkles disappear so that the surface of the film is smoothened) is proposed (JP 7-117328 A). However, in the method disclosed in JP 4-308798 A, wrinkles may be formed in a transfer sheet during the scale down of the sheet or a print pattern may deform without being scaled down uniformly. Therefore, it may be impossible to transfer a print pattern printed in a transfer sheet to an article precisely. In the case of the thin film for transfer printing disclosed in JP 7-117328 A, it may be impossible to form a high-definition print pattern on a thin film due to a reduced surface smoothness of the thin film, or when a film is floated on the surface of water, the film may be wrinkled due to the swelling property difference between the PVA and the natural gum-based mucilage. It, therefore, may be impossible to transfer print a high-definition print pattern. Moreover, it may be difficult to remove the natural gum-based mucilage during a step of washing the thin film after transfer and, therefore, it is not sufficient as a film with suppressed extensibility on the surface of water.
In addition, a method comprising applying an ink activating solvent after floating a transfer sheet on the surface of water is proposed (for example, JP 58-191187 A). According to this method, it is possible to control to some extent a spread of a print pattern caused by swelling of a transfer sheet. In this method, however, no considerations are given to control a time of floating a transfer sheet on the surface of water or a time passing after an ink activating solvent is applied until transferring it to an object. Therefore, the method cannot solve the problem in transferring a high-definition pattern.    Patent reference 1: JP 51-21914 A    Patent reference 2: JP 54-33115 A    Patent reference 3: JP 54-92406 A    Patent reference 4: JP 54-150208 A    Patent reference 5: JP 4-308798 A    Patent reference 6: JP 7-117328 A    Patent reference 7: JP 58-191187 A